In my external drive, I created a partition for bootcamp

[admin]

But in "Please select the destination disk" part, the only option it's giving me is what seems like the entire external drive.
It says:
Disk 1: WD easystore 2647

Which seems to be the name of my whole drive? (I have a portion of it named Bootcamp; it's empty and )

Does WintoUSB allow using only a portion of the drive as bootcamp or does it require the whole drive to be dedicated to bootcamp?

Or, since I have only the Bootcamp portion of my drive formatted to MS-DOS, will WintoUSB only affect that part of my drive and leave other partitions alone?

If you want to use a custom partition layout, you should select the "Keep the existing partition scheme" option, this option allows you to keep the existing partition structure without format the disk. In such case, you have to manually prepare the required partition, here is the guide for how to manually prepare partitions for portable Windows:
https://www.easyuefi.com/wintousb/resour...to-go.html